Microbubble Formulations for Photoacoustic Imaging and Sonothrombolysis Applications

Microbubbles provide a versatile platform for both ultrasound-mediated therapy and imaging. This dissertation outlines the design and characterization of two novel microbubble formulations used in separate photoacoustic imaging and sonothrombolysis applications.

Microbubbles as Photoacoustic Imaging Contrast Agents
Photoacoustic imaging is a noninvasive imaging technique that provides high contrast images of optical absorption deep within living tissue. Dual-modality ultrasound and photoacoustic imaging can simultaneously evaluate anatomical tissue properties with ultrasound and molecular properties with photoacoustics. The two modalities utilize the same ultrasound receive instrumentation, which provides inherently co-registered images and permits the development of contrast agents that respond to both light and sound for molecular imaging applications. In this dissertation, the design, synthesis, and imaging performance of light-absorbing microbubbles bearing gold nanorods on their surface was evaluated in vitro, in silico, and in vivo. Two responses to pulsed laser excitation were identified – in response to high laser energy, explosive boiling at the nanoparticle surface resulted in rapid microbubble expansion and high amplitude, non-linear photoacoustic emissions; in response to lower laser energies, photoacoustic emissions scaled linearly with increasing laser fluence, and finite element modeling predicted nanoscale microbubble radius oscillations at the microbubble resonance frequency. Both of these responses may be utilized to separate the photoacoustic signal derived from the light-absorbing microbubble from spurious background tissue signals, thereby increasing the specificity of the imaging technique and enabling novel molecular imaging approaches.

Microfluidic Production of Microbubbles for Sonothrombolysis Applications
Intravenous or catheter-directed administration of recombinant tissue plasminogen activator (rtPA) remains the standard of care for many thrombo-occlusive diseases, including ischemic stroke, deep vein thrombosis, and pulmonary embolism. However, fewer than 5 % of ischemic stroke and deep vein thrombosis patients receive thrombolytic therapy due to strict eligibility criteria and risk of severe bleeding. An alternative approach that has been developed to address the pressing need for improved recanalization techniques is the combination of rtPA with adjuvant ultrasound and microbubbles, a technique known as sonothrombolysis. Sonothrombolysis accelerates recanalization and has been investigated as a means to remove blood clots using lower doses of thrombolytic agents, thereby reducing the risk of severe bleeding and increasing patient eligibility for treatment. In this dissertation, the feasibility of a catheter-directed sonothrombolysis platform was evaluated in vitro and in vivo. Microbubbles were produced in real-time at the distal end of the catheter by a flow-focusing microfluidic device, and significantly improved thrombolysis rates were observed in both in vitro and in vivo models. In particular, the potential for a 3.33 ± 2.16 fold rtPA dose reduction was observed in a rat model of ischemic stroke, and neurological deficit scores were also significantly improved following sonothrombolysis treatment. Together, these results suggest catheter-directed sonothrombolysis techniques may accelerate recanalization and permit rtPA dose reduction, thereby addressing two pressing clinical needs.